Regarding the meaning of “open air assembly of persons”, after some searching I did find a court decision on this, by the U.S. District Court in Michigan. The case has to do with a Coast Guard helicopter that flew over a horse ranch and caused a rider to fall. I’ve copied the relevant bit :

"Plaintiffs also contend that the proper altitude benchmark is 1,000 feet because a total of twenty riders and spectators on a ten-acre site constitute an "open air assembly of persons." See 14 C.F.R. § 91.119(b). Plaintiffs' argument fails to recognize that this phrase must be construed in light of its regulatory context within the subsection dealing with "congested areas." Twenty people on a ten-acre site simply cannot be equated with a congested area of a city, town, or settlement. If the phrase were to be construed as Plaintiffs suggest, then a pilot could violate this regulation by flying below 1,000 feet over a backyard barbeque. This cannot be the intent of the regulatory language. The only testimony at trial was to the contrary. Smith Kalita testified that the phrase refers to crowds at stadiums, outdoor concerts, and the like. See, e.g., Barnum v. NTSB, 595 F.2d 869 (D.C.Cir.1979) ("crowds" around a monument constituted an open air assembly of persons)."

Full decision text is here:
https://asci.uvm.edu/equine/law/case.../mickalich.htm